The present invention relates to a new and improved method of preventing damage to the strand guide or roller apron elements of a continuous casting installation for metals, especially steel.
During the continuous casting of steel it sometimes happens that there arises undesired deformation of the strand, predominantly when operational disturbances of the continuous casting installation require interruption of the casting operation. If such shutdown of the casting operation is over a prolonged period of time, then the still thin strand shell or skin, due to inadequate strength, is not capable of withstanding the ferrostatic pressure of the liquid strand core or sump along unsupported sections of the strand, for instance, between pairs of guide rolls. In such case the strand shell or skin tends to bulge. If too great a time interval transpires until start-up of the continuous casting installation, during which time interval the solidification of the strand continues, then frequently it is no longer possible to again push back such bulged portions against the resistance of the partially solidified strand, by means of the guide rolls. Consequently, during the withdrawal of the strand it can bind or remain stuck between the guide rolls. This phenomenon is associated with overloading of such guide rolls and the elements of the support structure, and with constant withdrawal force, can lead to deformation or destruction thereof.
A further disturbance in the normal casting operation, which cannot always be avoided, is metal break-out, at least at a region of the first peripherally solidified strand directly below the mold. As a general rule there are associated therewith undesirable phenomena, since frequently a large quantity of steel outflows into the secondary cooling zone, solidifying at the parts of the installation located thereat, such as the support and guide rolls, the elements of the support structure and so forth. When this happens there almost always occurs appreciable damage requiring expensive repair work. The phenomenon of metal break-out, in the most general situations, requires interruption of the continuous casting operation.
The consequences of such shutdown of the continuous casting operation, apart from the time-consuming elimination of the direct cause of the metal break-out, usually resides, in particular, that there arise irreparable deformations of the strand guide rolls. Such is caused by one-sided, local overheating of the rolls which come into contact with the stationary, hot strand or casting. Upon restarting of the continuous casting installation, especially upon again placing into operation the strand guide arrangement, such roll deformations can result in out-of-true rotation of the rolls, and furthermore, due to irregular loading of the bearings of the rolls or rollers such can cause destruction of such roll bearings or roll fracture. Moreover, out-of-true running of the guide rolls, due to deformation of oppositely situated rolls in the same sense, can result in the periodic application of irregular forces at the strand. At locations of intensified pressure there can arise the phenomenon that the strand will be rolled or reduced, associated with undesirable fissure formations at the strand.
In practice attempts are made to avoid the above-explained disadvantages which arise upon interruption of the strand withdrawal operation, by performing the following technique. Upon interruption of the infeed of the molten steel to be cast into the continuous casting mold, then the strand is completely withdrawn by means of large dimensioned withdrawal units without interruption, against the forces hindering the withdrawal of the strand. These obstructing forces result from irregularly formed strands or strands which have welded together with parts of the continuous casting installation. When carrying out this procedure it can happen that, owing to the presence of extremely strong welding of the strand with the roller apron elements, such as the rolls, grids and so forth, and which welds can no longer be broken by simply being torn-away during the process of withdrawing the strand, the large withdrawal forces tend to act directly upon the support and bearing elements of the strand guide arrangement or roller apron. Upon overloading of such support and bearing elements of the strand guide arrangement permanent deformation and damage thereto can arise.
Thus, for instance, if a so-called first zone of the strand guide arrangement or roller apron is cast due to overflowing steel, the aforementioned overloading can result in an undesirable deformation of the support arms and the support journals of the first zone, and thus, can lead to deviations in the geometry of the strand guide arrangement or roller apron. This, in turn, requires expensive, time-consuming repair work. An advantage of the above-described method, which strives to prevent stoppage of the strand in the presence of a metal break-out, in contrast to conventional casting techniques where the strand is allowed to cool, therefore is no longer realized when such situation occurs. Deviations in the geometry of the strand guide arrangement, which cannot be immediately detected, moreover lead to the formation of fissures at the cast strand or casting, and thus, reduce the quality of the cast strand.